The invention relates to opthalmic testing instruments and more particularly to a device for measurement of visual acuity.
Typical clinical methods for measuring acuity involve the use of wall charts containing a fixed array of Snellen letters, Tumbling E targets or other accepted acuity targets. The patient ordinarily views the charts from a fixed distance (usually 20 feet). A second method in common use involves the projection of targets onto screens placed at a fixed distance by means of an optical system. Other methods, such as rear illuminated fixed target displays and motorized projectors utilizing a fixed series of target slide materials are also used.
The determination of visual acuity is an essential part of every eye examination. During the course of such an examination, acuity may be measured repeatedly to ascertain the resolution of each eye independently and both eyes together. The examination may also consist of independent and combined testing of the eyes with the aid of corrective lenses. In fact, the repeated determination of acuity forms an essential part of the process of refracting or determining the optimal corrective lenses to alleviate ametropia as well as a means for assessing the progress of ocular patholgy.
The methods and devices described above and presently in use have inherent defects and inadequacies which affect the validity of the data and the productivity of the examiner.
For example, memorization of the target materials by the patient is known to be one problem affecting the ability of the clinician to ascertain the patient's acuity. Acuity measurement devices which present a fixed array of targets are easily memorized, either voluntarily or involuntarily.
While not considered to be a critical aspect of testing, the inability to test acuity in small increments between standard target testing sizes is viewed as a restriction in most presently used methods.
The effect of ambient light upon the luminance and contrast of the projected type displays is considered a limitation because it requires that acuity be tested in reduced illumination. Hence the acuity of some patients measured in the clinic may differ considerably from their acuity in non-clinical environments.
The inability of known devices to provide white on black as well as black on white targets, to more adequately test low vision patients, is considered to be a disadvantage. The inability to present a single or multiple targets to compare "crowded" to "uncrowded" acuity for the detection of early amblyopes is a further disadvantage.
Finally, time has become a critical resource for the practitioner. The time required to instruct the patient regarding the target he is to view, or the time the clinician requires to find, adjust and align the required target in the case of projection devices becomes increasingly important with heavy patient loads.